Hydrocarbon compounds are base materials of the chemical industry and starting materials for a multitude of secondary products. Typically, the hydrocarbon compounds are obtained in the primary production processes in the form of mixtures which by means of separation processes—above all the fractional distillation—must be separated into individual fractions or pure substances. The interconnection of the separators used in the conventional processes leads to large dimensions of the individual equipment parts and to a high specific consumption of operating materials.
Accordingly, the optimum design of the separation process is of high importance. The hydrocarbon compounds should be produced in rather pure form without the presence of oxygen-containing organic compounds (oxygenates). Oxygenates are understood to be compounds which exclusively are composed of carbon, hydrogen and oxygen; in general, these are alcohols or ethers, which can also be admixed to the gasoline.
In particular in the olefin synthesis by reaction of oxygenates such as methanol and/or dimethyl ether (DME) on molecular sieve catalysts, as it is described for example in the European Patent Applications EP 0448000 A1 and EP 0882692 A1 and the European Patent EP 1289912 B1, it is the object to remove small amounts of the oxygenates used as educt, such as methanol or dimethyl ether, from the reaction product, a hydrocarbon stream rich in olefins, in an efficient way, since the oxygenates are catalyst poisons in the succeeding further processing of the olefins to polyolefins. In Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Polyolefins”, chapter 2.3.1 “Propene”, limit values are indicated for the content of oxygenate components in a propylene stream with polymerization purity (so-called “polymer grade propylene”). For example, the tolerable concentration of methanol in propylene with polymerization purity maximally amounts to 5 vol-ppm.
For separating smaller amounts or lower concentrations of oxygenates to achieve polymerization purity, adsorptive processes also are suitable. The same are also employed as downstream separation processes for example after a distillative separation of oxygenate, in order to definitely comply with the required limit values.
In the processes taught in the above-mentioned European patent applications and patents, the olefin synthesis is carried out in fixed-bed reactors with one or more catalyst beds. With increasing deactivation of the catalysts used, the oxygenate concentration in the olefin product stream also rises continuously, so that the separation process to be used must also be suitable for time-varying oxygenate concentrations in the feed stream to be treated.
According to the prior art, the oxygenates are separated from the hydrocarbon compounds by a classically connected distillation or a physical wash. In particular in large-scale industrial plants this is costly and expensive. The documents WO 03/020671, WO 03/020672 and WO 03/020678 describe processes for the extractive distillation of olefins.
The U.S. Pat. No. 7,678,958 B2 describes a process for removing DME from an olefin stream by means of distillation and a downstream water wash, which was obtained in the conversion of oxygenates to olefins (OTO). The process includes the distillation of the olefin stream, so that DME together with propane is obtained as bottom product and thus removed from the olefin stream. The olefin stream then can be supplied to one or more further distillation stages, in order to ultimately obtain an ethylene stream and a propylene stream with polymerization purity, wherein the DME concentration in both streams does not exceed 10 wt-ppm. The DME is removed from the propane stream by water washing, and the separated DME is recirculated to the OTO reactor.
The German Patent Specification DE 102004052658 B3 teaches a process for removing oxygenates from hydrocarbon mixtures, in which the remaining amount of oxygenates in the olefin stream is reduced to below 1 ppm, and a separation into partial fractions is achieved, wherein the apparatus dimensions and the specific consumptions of operating materials are minimized. The solution of this object is achieved in that the mixture of hydrocarbons and oxygenates is processed in a two-stage separation process. The feedstock hydrocarbon mixture is present in a two-phase form, wherein the heavier hydrocarbons are present in the liquid phase. The two phases are not charged together to one distillation column as conventionally, but are each guided separately into two distillation columns. From the liquid phase in the first column a light fraction is separated, in which the olefin product and oxygenates are contained. The gas phase is charged to the second column together with the light fraction of the first column. This second column is an extractive distillation column.
The mixture is separated into a light and a heavy hydrocarbon cut. In the process, a solvent is supplied into the upper part of the column, which dissolves the oxygenates. As a result, the content of oxygenates is distinctly decreased as compared to the prior art. Various components are taught as solvent, such as the alcohol methanol used as educt in the preceding methanol-to-propylene process (MTP® process). It is advantageous that thus a substance inherent to the process is used as solvent, which is available anyway, and the separated oxygenate can be recirculated to the olefin synthesis together with a residual content of methanol.
The distillative processes for oxygenate separation as taught in the prior art have in common that the oxygenate to be separated is obtained with a hydrocarbon fraction with similar boiling point, which subsequently must be separated from the hydrocarbon fraction by a further step, for example by an additional distillation step, an extractive distillation step, by washing or by adsorption or by a combination of several of these measures.